1. Field of the Invention
This invention relates to a rotor of a buried magnet-type permanent magnet synchronous rotary electric motor. The invention, further, relates to a motor mounting the above rotor and to a machine tool mounting the above motor.
2. Description of the Related Art
A buried magnet-type synchronous motor has been widely used. Such a motor is a built-in motor which is directly incorporated in, for example, a main shaft of a machine tool to directly drive the main shaft, and is required to have a relatively large inner diameter and to rotate at a high speed.
The rotor of the built-in motor is fixed to the outer diameter portion of the main shaft, usually, by shrinkage fit. Therefore, the inner diameter of the rotor of the built-in motor is required to be nearly the same as the outer diameter of the main shaft.
Depending upon the dimensional precision and surface roughness of a workpiece that is to be machined, the main shaft of the machine tool requires a machining precision on the order of micrometers. For example the main shaft having a low rigidity undergoes vibration affecting the dimensional precision, surface roughness and appearance of the workpiece that is machined. This is attributed to low performance of the main shaft.
The rigidity of the main shaft is determined depending, for example, upon the diameter of the main shaft. Usually, the geometrical moment of area of the main shaft increases with an increase in the diameter thereof and, as a result, the main shaft has increased rigidity. Therefore, it is desired that the main shaft has a diameter which is as large as possible within a range permitted by a maximum rotational speed of the main shaft.
As described above, the rotor of the built-in motor has been directly attached to the main shaft by shrinkage fit. Therefore, the rotor of the built-in motor, is also required to have a relatively large inner diameter as the diameter of the main shaft.
In this connection, it is presumed that a hollow cylinder that serves as the rotor of the built-in motor rotates about the center axis thereof. In the case of a cylinder having the same outer diameter, the stress that generates in the cylinder increases with an increase in the inner diameter thereof. In addition, the maximum rotational speed which the rotor can withstand decreases with an increase in the stress. The same also holds for the rotor of the buried magnet-type synchronous motor; i.e., maximum stress that generates in various parts increases and a maximum rotational speed that can be permitted decreases with an increase in the inner diameter.
Therefore, as described above, the rotor of the buried magnet-type synchronous motor, specifically, the rotor of the built-in type motor incorporated in the main shaft of a machine tool, is required to satisfy the requirements of both high rotational speed and large inner diameter. However, as described above, an increase in the inner diameter is accompanied by an increase in the stress that generates due to the rotation. Therefore, it is necessary to decrease the maximum rotational speed that can be permitted to rotate. In other words, an increase in the rotational speed and an increase in the inner diameter constitute conflicting technical elements due to their own natures.
FIG. 17 is a transverse sectional view showing a representative shape of magnet slots of the buried magnet-type synchronous motor according to a prior art. In FIG. 17, eight poles comprising eight magnets M are arranged in a core 110 in the circumferential direction thereof. As shown in FIG. 17, the magnets M are arranged in the circumferential direction in a manner that the magnetic poles alternately appear like NSNS - - - . In FIG. 17, each pole is constituted by a row of magnets in the axial direction.
FIG. 18 is a partially enlarged view illustrating a pole of the motor shown in FIG. 17. As shown in FIG. 18, in general, one magnet slot 300 corresponds to one magnetic pole. The magnets M corresponding to each magnetic pole have their axial magnetic poles arranged in a row in the same direction, i.e., arranged in one magnet slot 300.
However, as the rotational speed increases, the centrifugal force also increases, and core portions at both ends of the magnets M have to bear increased strength. If the stress produced by the rotation exceeds an allowable value, then the core 110 breaks. Therefore, a limit is imposed on increasing maximum rotational speed.
To avoid this, it has been attempted to divide the magnet slot in the same pole into a plurality of magnet slots, and to arrange a core in a region between the divided magnet slots. FIG. 19 is another sectional view showing a typical shape of magnet slots in a buried magnet-type synchronous motor according to prior art. In FIG. 19, the two magnets M1 and M2 arranged in the divided magnet slots 310 and 320 constitute one pole. A core portion 120 is provided between the magnet slots 310 and 320 in which the magnets M1 and M2 are arranged, respectively. In FIG. 19, the core portion 120 increases the strength against the centrifugal force. Therefore, provision of the core portion 120 shown in FIG. 19 is sufficient when the maximum rotational speed is not very high, the inner diameter of the rotor does not have to be increased, and the stress is concentrated to a small degree.
However, with this shape, if it is required to further increase the speed or to further increase the inner diameter, then the stress further increases in the portions on the inner side of the core portions 120 in the radial direction and in the core 110 near the ends of the magnets M1 and M2, and the limitation is readily reached.
In order to solve the above problem, Japanese Unexamined Patent Publication No. 2002-281700 discloses a shape of magnet slots of the rotor of a buried magnet-type permanent magnet synchronous motor by giving attention to suppressing the stress at the time of high-speed rotation. Japanese Unexamined Patent Publication No. 2002-281700 discloses the shape of a core portion extending in the radial direction between the magnets of the same pole (hereinafter referred to as core portion between the magnet slots of the same pole) and the shape of vicinities thereof. According to Japanese Unexamined Patent Publication No. 2002-281700, the concentration of stress is suppressed by the core portion, and a maximum rotational speed can be increased.
When the motor rotates, the outer core portion located on the outer side of the magnet in the radial direction produces a load due to the centrifugal force generated by the mass of the magnet and the mass of the outer core portion itself. The magnitude of the load increases with an increase in the rotational speed.
In the shape of slots disclosed in Japanese Unexamined Patent Publication No. 2002-281700, the core portion between the magnet slots of the same pole shares the load produced in the radial direction by the centrifugal force. Therefore, it is possible to decrease the stress generated in the outer circumferential portion at both ends of the magnet and, as a result, to increase the rotational speed. In other words, the method disclosed in Japanese Unexamined Patent Publication No. 2002-281700 offers a certain effect for the motors that rotate at high speeds.
As described, it is often required to provide a rotor that has a further increased inner diameter and that rotates at a high speed.
If the structural strength of the rotor is considered, a core portion which is on the inner side of the magnet slot in the radial direction serves as an important portion supporting the strength. In other words, the centrifugal force due to the magnet and the core portion on the outer side of the magnet slot in the radial direction, is exerted as a load on the core portion on the inner side of the magnet slot in the radial direction.
A large inner diameter means that the core portion on the inner side in the radial direction has a small thickness in the radial direction. In other words, the structure (=core portion on the inner side in the radial direction) on which the centrifugal force finally acts) has a small sectional area. Therefore, stress is specifically high in the core portion on the inner side in the radial direction.
As described above, an increase in the inner diameter of the rotor results in a decrease in the thickness of the structure that supports the stress produced by centrifugal force. Specifically, the thickness of the core further decreases in the radial direction near the core portions on the inner side of the magnet slots in the radial direction and near the core portions between the magnet slots of the same pole. Therefore, such portions have the smallest sectional area for supporting the load produced by the centrifugal force due to rotation. Accordingly, the effect of stress distinctly appears in these portions as the inner diameter increases.
Therefore, the shape disclosed in Japanese Unexamined Patent Publication No. 2002-281700 is disadvantageous when a large inner diameter is required. Specifically, the distance from the inner circumferential surface of the core in the radial direction becomes the smallest near a corner that connects the outer edge portion of the core portion between the magnet slots of the same pole to the edge of the magnet slot on the inner side in the radial direction. In such portions, the stress easily increases if the inner diameter increases. The shape disclosed in Japanese Unexamined Patent Publication No. 2002-281700 is not sufficient for increasing a maximum rotational speed on account of the reasons mentioned below.
In Japanese Unexamined Patent Publication No. 2002-281700, the core portion between the magnet slots of the same pole has its width varying from the inner circumferential side toward the outer circumferential side as it goes, for example, from the inner circumferential side toward the outer circumferential side in the radial direction.
In other words, the width is small (narrow) at some portions and is large (wide) at other portions of the core portion between the magnet slots of the same pole. The narrow portion of the core portion between the magnet slots of the same pole suppresses the magnetic flux that passes through the core portion and closes the loop without contributing to generating torque, i.e., suppresses the leakage flux. On the contrary, the wide portions of the core portion between the magnet slots of the same pole have the magnets contacting thereto, thus, it is possible to determine the positions of the magnets in the direction of rotation, so that the magnets do not move while rotating.
In Japanese Unexamined Patent Publication No. 2002-281700, the narrow portions and the wide portions are alternately provided in the core portion between the magnet slots of the same pole in the radial direction.
In an embodiment of Japanese Unexamined Patent Publication No. 2002-281700, for example, there are arranged a first narrow portion, a wide portion and a second narrow portion in this order on the core portion between the magnet slots of the same pole in the radial direction from the inner side thereof to the outer side thereof in the radial direction.
Further, in order to reduce the stress, an arc portion must be provided between the narrow portion and the wide portion. In general, the arc portion having a large radius is more effective in reducing the stress. According to the prior art, the core portion between the magnet slots of the same pole is formed indented and, therefore, the arc portion must be formed at the portions connecting the narrow portion to the wide portion. Therefore, a number of arc portions must be provided within a size that substantially corresponds to the thickness of the magnet. Accordingly, limitation is imposed on the radii of the arcs. Depending upon the thickness of the magnet, further, it is not often necessary to provide an arc portion of a size large sufficient for reducing the stress.
As a target maximum rotational speed increases, a larger stress is produced and the radius of the arc portion must be increased to relax the stress. In this regard, according to Japanese Unexamined Patent Publication No. 2002-281700, a maximum diameter of the arc that can be attained is determined depending upon the thickness of the magnet. Under this limitation, the upper limit of a maximum rotational speed of the rotor is determined by a maximum stress produced by the arc of a maximum diameter that is attained.
Further, according to Japanese Unexamined Patent Publication No. 2002-281700, the arc provided in the outer edge of the core portion between the magnet slots of the same pole has nearly the same size either on the side close to the rotary shaft or on the side close to the outer circumference. However, in the case of the rotor having a large inner diameter, a particularly increased force is exerted on the core portion of the magnet slots on the side of the rotary shaft, and the stress as a whole tends to increase in the core portion on the side of the rotary shaft. Therefore, even a slight change in the shape causes the stress to be easily concentrated. Therefore, consideration, must be given to concentration of stress by, for example, increasing the size of the arc close to the core portion on the side of the rotary shaft to be larger than the size of the arc provided in the outer edge of the core portion between the magnet slots of the same pole.
The present invention was accomplished in view of the above-mentioned circumstances, and provides a rotor which is capable of rotating at a higher speed maintaining stability as a result of suppressing the stress near the bottom edge of the magnet slot on the outer edge of the core portion between the magnet slots of the same pole, provides a motor mounting the above rotor, and provides a machine tool mounting the above motor.